posted August 02, 2013 09:26 AM
A transit probably triggered a natal hard aspect.
There are various indicators for mental 'illness' (I don't like that word) - mainly hard aspects to the Moon and Mercury, but could be any other hard aspect that affects the mental / emotional level.
So look at the transiting outer planets aspecting an afflicted natal planet (especially if it rules or resides in the 1st, 6th or 12th house).